Conventional power converters use an analog error amplifier, compensated to provide the desired dynamic response, in conjunction with a ramp driven comparator to generate the switching control waveforms for the power switching transistors. Digital power converter controllers have been developed that are based on a digitization of this analog control architecture. This class of controller, although implemented with digital circuits, is capable of generating a continuously variable duty ratio. The digital control architecture described herein is based on a duty ratio quantization technique where the duty ratio of the power transistors is only allowed to take on certain discrete values. To provide dynamic regulation, the duty ratio value is updated each switching cycle by performing calculations on data obtained by sampling the output voltage of the power converter with a digital error amplifier.
Digital control based on duty ratio quantization offers several advantages over conventional analog control. Since digital filter techniques are used for dynamic regulation, and the sampling frequency of the digital filter is equal to the switching frequency of the power converter, the quantized duty ratio digital controller may be used at any switching frequency without requiring recompensation. Dynamic regulation characteristics are easily altered by the selection of digital weighting coefficients. The basic digital controller is easily modified to include special functions such as output current limiting and soft-start. Computer simulation may easily be accomplished since the action of the power switching transistors is always defined one cycle in advance.
A theoretical disadvantage of duty ratio quantization is that low frequency quantization noise is produced in the output of the converter. However, if a sufficiently small quantization level is used for duty ratio generation, the quantization noise may be reduced to a level well below the switching ripple.
Power converter controllers that use digital proportional-integral-differential (PID) feedback are shown in the following papers:
N. R. Miller, "A Digitally Controlled Switching Regulator," PESC Record, 1977. PA1 V. B. Boros, "A Digital Proportional Integral, and Derivative Feedback Controller for Power Conditioning Equipment," PESC Record 1977. PA1 H. Matsuo and F. Kurokawa, "Regulation Characteristics of the Digitally Controlled DC-DC Converter," PESC Record, 1983. PA1 T. V. Papathomas and J. N. Giacopelli, "Digital Implementation and Simulation of an Average Current Controlled Switching Regulator," PESC Record, 1979.
These schemes use a voltage-controlled-oscillator to measure the error in the signal output that is to be regulated. Hence, the duty ratio is a continuous variable and varies in response to changes in the average output voltage from one switching cycle to the next.
In R. Bruckner and I. Khamare, "Optimizing Converter Design and Performance Utilizing Micro controller System Feedback and Control," Proceedings Powercon 8, 1981, a digitally controlled power converter is described that uses a quantized duty ratio technique but not PID control. With this type of controller, the output signal is sampled with an analog-to-digital converter and the duty ratio is calculated one cycle in advance based on the sampled data.
The digitally controlled power converter of the present invention uses both PID control and duty ratio quantization. PID control is desirable because it offers very good regulation. In addition to good regulation characteristics, a practical power converter for many applications must have output current limiting, soft-start, undervoltage lockout and overvoltage shutdown. The digitally controlled PID power converter of the present invention has all of these special functions. The soft-start function changes the digital weighting coefficients of the controller during power-up. The only external signal that is required is a logic "edge" to initiate the function. This method is not practical with analog control because it would require many extra components. None of the digital control systems descibed in the above-noted papers are implemented in the manner of the present invention where the digital controller serves as ripple-current regulator when any of the outputs of the power converter are overloaded.
The Brackner, et al. paper, describes a digital overcurrent protection scheme, but in this device, the output current is not regulated using the control loop that is already in place as it is in the present invention, and furthermore, multiple output supply current limiting is not disclosed in Brackner, et al.
The described controller also has undervoltage lockout and overvoltage shutdown functions. These functions cause the controller to be "cleared", thereby forcing the duty ratio to go to zero as long as the input voltage is too low, or any of the output voltages are too high. None of the power converters described in the above noted papers have either of these functions.